I think that jgenie's suggestion of finding a workout partner to keep both of you accountable is a good one.
When I did charity walk weekends, I would volunteer to lead training walks during the lead-up period to ensure I was walking regularly. It helped me meet new people (I am very introverted IRL) and we kept each other accountable while training.
To make something like this work over distance, maybe make a standing calendar appointment with a friend and call each other at the start of your walk time?
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Liz
DD (3/2010)
"Make mistakes! Get messy!" - Miss Frizzle